1. Field of the Invention
The present invention relates to a cycle wheel spoke and a cycle wheel comprising such a spoke.
2. Background Information
Conventionally, the spokes of a cycle wheel are tensioned when the wheel is assembled because, due to their length and small cross section, they cannot work in compression. When the cycle is in use, the spokes are subject to fatigue development, as they undergo a load cycle with each turn of the wheel, thereby constantly varying their tension. Indeed, when a spoke is located between the ground and the wheel hub (such as positioned vertically in relation to the ground), its tension is reduced due to a slight deformation of the wheel caused by the cyclist's weight which provides a force countering tension; and when a spoke passes above the hub (again, such as positioned vertically in relation to the ground), it is re-tensioned by the pre-stressing and the load effect. The ends of the spokes, each having a fastening zone enabling them to be assembled to the rim or the hub of the wheel, are more affected by the fatigue phenomenon than the central portion of the spokes, due to the stress concentrations caused by the geometric variations of the fastening zones (thread, elbow, head, etc.), and therefore have a greater risk of breakage. Therefore, the ends of the spokes must have higher mechanical properties than those of the central portion of the spokes, which is why their cross section is conventionally greater than that of the central portion.
In addition to a very good tensile strength, the spokes must have a certain degree of flexural flexibility. A flexural flexibility of the spokes provides the entire wheel with greater resistance to side impacts, such as those encountered, for example, when a cyclist riding in a very compact group of riders inadvertently moves too close to another cyclist and the spokes of his front wheel engage the rear derailleur of the cyclist immediately ahead of him/her. Such engagement is capable of breaking a plurality of spokes, which can cause the front wheel to collapse and the cyclist to lose control of the bike, thereby resulting in a very serious fall. When a spoke has a certain flexibility, this type of engagement may damage the spoke without causing it to break.
Spokes made of a metallic material, such as steel, can combine good tensile strength and flexibility, but these spokes are heavy.
Spokes made of a composite material are, for equal strength, much lighter but more fragile. In particular, composite materials have exceptional tensile properties, but also very low shear properties, which does not make it possible to provide efficiently working threads on composite rods. Therefore, these materials are very well suited to the central zone which is subject to tensile stress, but it is very difficult to fasten these spokes to the rim and the hub without using a fastening metal piece specifically for adjusting their tension independently of one another, in order to ensure a balanced geometry in the wheel.
It is known to combine metallic and composite materials to make hybrid spokes.
For example, the document EP 1 930 146 describes a spoke comprising an aluminum core covered with a sheath formed of carbon fibers coated with polymerization-cured resin.
This spoke has the advantage of being very lightweight, due to its aluminum core. However, despite the use of a composite material, this spoke is not necessarily suitable for applications requiring very high rigidity, because the modulus of elasticity, and therefore the rigidity of the aluminum, is only on the order of 71,000 MPa. In addition, a spoke of this type requires specific and quite large fasteners on the rim and the hub, and such fasteners are not adaptable to all types of rims/hubs.
Another example of hybrid spoke is described in the document FR 2 915 710. In this case, the core is formed by a blade and two end pieces welded together. Given that welding locally deteriorates the properties of the metal, it is necessary to take such deterioration into account when sizing the core. Finally, obtaining a spoke that is adequately strong requires making a relatively heavy core. Moreover, a spoke having a core that requires three distinct elements assembled to one another is expensive to manufacture.
A third example of a hybrid spoke is provided in the document EP 1 420 963. The spoke illustrated in FIGS. 1 to 6 of this document comprises a metallic core having a constant cross section and a sheath made of composite material. The objective of the designer of such a spoke is to improve the aerodynamics, and he only provides a wing shape to the sheath.